The present invention relates to an ink jet recording apparatus wherein sub-tanks for supplying ink to a recording head are mounted on a carriage, and relates in particular to an ink jet recording apparatus that can control, within an appropriate range, the quantity of ink retained in each sub-tank and to a method of supplying ink the sub-tanks of the ink jet recording apparatus.
A related ink jet recording apparatus of a serial printing type is provided with: an ink jet recording head, which is mounted on a carriage and is moved in a width direction of a recording sheet; and a paper feeding unit, for moving the recording sheet in a direction perpendicular to the width direction in which the recording head is moved. Based on printing data, the ink jet recording apparatus, when printing, ejects ink droplets from the recording head onto a recording sheet.
For this type of recording apparatus, which is provided for office or for professional use, ink cartridges having large capacities must be provided so that they can cope with the printing of a comparatively large quantity of data. Therefore, a recording apparatus wherein main tanks serving as ink cartridges are loaded into cartridge holders arranged on the main body of the apparatus, for example, is provided.
In a thus arranged recording apparatus, sub-tanks are located on a carriage on which a recording head is mounted, and ink in the sub-tanks which supply ink to the recording head is replenished via ink tubes extending from the main tanks to the sub-tanks.
Recently, the demand has increased for large recording apparatuses that can be used for the printing of data on large sheets of paper and that provide extended scanning distances for carriages. For such a recording apparatus, in order to improve throughput, number of nozzles formed in the recording head is attempt to be increased. Further, also in order to improve throughput, such a recording apparatus is requested that during the performance of the printing process, the main tanks can, as needed, concurrently replenish the ink in the sub-tanks mounted on the recording head carriage, so that a stable supply of ink for the recording head can be provided by the sub-tanks.
In this recording apparatus, the length of an ink tube used to connect a main tank and a sub-tank is naturally extended. Further, as is described above, since an increased number of nozzles are formed in the recording head, ink consumption is increased, the dynamic pressure on the ink passing through each ink tube communicating with a main tank and a sub-tank is increased. Accordingly, as a technical object, the ink supply available for replenishing the sub-tanks falls short of that which is actually required.
For the resolution of this object, the present inventor proposes an appropriate recording apparatus. In this apparatus, by introducing air under pressure into a main tank, the flow of ink from the main tank to a corresponding sub-tank is forcibly generated, and a required and adequate quantity of ink is supplied to replenish the sub-tank. In this case, ink level detector must be arranged in the sub-tank in order to constantly maintain the volume of the ink stored therein within a predetermined range. By using result off the detection by the ink level detector (hereinafter also referred to as surface detector), an ink supply valve that is located along an ink path leading from a main tank to a sub-tank is controlled to be opened and closed. This configuration has also been proposed by the present inventor.
Preferably, an ink level detector includes: a permanent magnet attached to a float member stored in a sub-tank; and a Hall device which is positioned on the side wall of the sub-tank to detect and measure the magnetic force of the permanent magnet. With this arrangement, a low ink state in which the quantity of ink retained in the sub-tank is less than a predetermined value, or a full ink state, in which the quantity of ink retained in the sub-tank is equal to or greater than the predetermined value, can be detected by utilizing the output of the Hall device.
When the ink level detector detects the low ink state, the ink supply valve located along the ink path leading from the main tank to the sub-tank is opened, so that replenishment of the ink in the sub-tank can be effected. When the ink level detector detects a full ink state, the ink supply valve is closed, thereby halting the replenishment process. By repeatedly performing this process, the volume of the ink retained in a sub-tank can be maintained within a predetermined range.
The ink level detector is employed to supply or halt the supply of ink to the sub-tank, the following process is repeated, when the low ink state is detected, the ink is immediately supplied to the sub-tank, and when the full ink state is detected, the supply of ink to the sub-tank is halted. That is, since, during printing, a low ink state can be detected and the replenishment of ink effected after only a small quantity of ink has been consumed, and since a full ink state can be detected after only a small quantity of ink has been supplied, the on and off supply of ink is rapidly and frequently, cyclically repeated.
As one example problem that may arise when a recording apparatus is thus arranged, as the carriage reciprocates, rippling of the ink in the sub-tanks mounted on the carriage may occur and may result in the available ink volumes being erroneously detected, an unintended and undesirable condition that may also be encountered when for some other reason vibration of the recording apparatus occurs. Therefore, if due to this effect low ink states are erroneously detected, even though the sub-tanks are actually filled, ink replenishment process is performed, and it may cause the sub-tanks to overflow, and may in some cases precipitously produce a critical problem by causing ink to leak from the sub-tanks.
FIGS. 9 and 10 are specific diagrams showing an example in which the ink level detector has erroneously and unpropitiously detected the ink level in a sub-tank. First, in the example in FIG. 9, one Hall device has been provided to detect the strength of the magnetic field of a permanent magnet attached to a float member. With this arrangement, when only a small quantity of ink remains in the sub-tank, the strength of the magnetic force acting on the Hall device is very weak. In this case, as is indicated in region (1) in FIG. 9, the Hall device is OFF, identifying a low ink state, and the operation for supplying ink to the sub-tank is performed.
As the ink supply operation raises the level of the ink in the sub-tank, the float member is accordingly raised, until a predetermined strength is attained by the magnetic force of the permanent magnet to the Hall device and renders the Hall device ON, thereby identifying a full ink state, which in FIG. 9 is indicated by a shaded region. When the full ink state is identified, the supply of ink to the sub-tank is halted. After which, if rippling of the ink in the sub-tank occurs, as is described above, due to the reciprocation of the carriage during printing, or as the result of other vibrations, the detector may detects region (2) shown in FIG. 9.
In this case, since the magnetic force acting on the Hall device is reduced, the Hall device is rendered OFF, erroneously detecting a low ink state. Therefore, re-supply of ink to the sub-tank is performed, and an excessive quantity of Ink flows into the sub-tank. Therefore, since the Hall device maintains to identify OFF state, ink leaks from the sub-tank, thereby producing a critical problem.
To eliminate this problem, two Hall devices may be provided in the direction in which the permanent magnet moves (vertically). This arrangement is shown in FIG. 10. As is pictured in FIG. 10A, the magnetic force detection regions of the upper and lower Hall devices are overlapped. Region U is defined so that the upper Hall device is rendered ON indicated, and the region L is defined so that wherein the lower Hall device is rendered ON indicated. With this arrangement, the ink level in the sub-tank can be detected for four states, upper OFF/lower OFF, upper OFF/lower ON, upper ON/lower ON and upper ON/lower OFF, in consonance with the combined outputs of the upper and lower Hall devices.
However, because of variances in the magnetic detection sensitivity exhibited by the Hall devices, and variances due to assembly errors in the distances between the permanent magnet and the Hall devices, the magnetic detection regions of the upper and lower Hall devices may not overlap, as is shown in FIG. 10B. In the state shown in FIG. 10B, a region (3), whereat both Hall devices are rendered OFF, is generated between the magnetic detection regions of the Hall devices, which are depicted as shaded portions.
Therefore, in a full ink state has been detected, since printing, and reciprocation of the carriage, or other vibration sources, causes rippling of the ink in the sub-tank, the region (3) is generated and a low ink state is erroneously detected. In this case, since ink is supplied to the sub-tank due to the error, a sub-tank overflow occurs, and the printing must be halted to perform maintenance.
As is described above, when an ink level detector provided with a Hall device and a permanent magnet attached to a float member is employed, the first problem that occurs is that the supply of ink to the sub-tank is frequently repeated, and the second problem that occurs is that vibration causes the quantity of ink in the sub-tank to be erroneously detected.